LDL-receptor cycle

LDL-receptor cycle

Following synthesis, the nascent LDL receptors are transported to the cell surface via the Golgi complex. At the plasma membrane, they quickly cluster into specialized regions of the plasma membrane known as clathrin-coated pits. ApoB-100 or apoE-containing lipoproteins that bind to LDL receptors are endocytosed within clathrin-coated pits. By a mechanism of vesicle budding and redirection, the LDL-R is recycled to the cell surface, whereas the lipoproteins are directed to lysosomes where they undergo hydrolysis. The liberated cholesterol is then used by the cell for the synthesis of plasma membranes, bile acids, and steroid hormones, or stored in the ester droplet form.



The cholesterol also triggers a number of important regulatory mechanisms including suppression of both endogenous cholesterol synthesis and the expression of the LDL-R itself. Cellular cholesterol content is the major LDL receptor regulator.

Different mutations in the LDL-R gene that give rise to the FH phenotype have been shown to affect different steps in the LDL-R pathway, including synthesis of the LDL-R, transport of the LDL-R to the cell surface, clustering of the LDL-R in coated pits and subsequent receptor internalization, and the ability of the LDL-R to bind to its ligands and to recycle.



Cholesterol uptake in the form of LDLs and processing in tissues, involves all the following steps or enzymes EXCEPT 


LDL binds to its receptor. Which of the following statements about this is true?